Vehicle brake systems generally have a nonrotational frictional member which may be placed in frictional engagement with a rotational member upon the application by the user of a brake actuator. The pressure applied by the frictional member upon the rotational member is proportional to the pressure applied by the user upon the brake actuator.
Prior art discloses systems to assist in the application of additional pressure either upon the brake actuator or directly to the frictional member. Some systems have used engine vacuum to enhance the pressure, applied either hydraulically or mechanically, to the frictional member.
Upon the application of the frictional member, brake pads, to the rotational member, brake disk or drum, the brake pads and housing experience a torque. This torque has been utilized by some systems to create additional hydraulic or mechanical pressure on the brake pads.
Safety considerations have limited what feedback systems may be utilized to transmit the torque into additional brake pressure. An open or negative feedback loop it totally unacceptable as the increased brake pressure simply creates more brake pressure until the vehicle is stopped. If the loop cannot be broken, the brakes cannot be released.
U.S. Pat. No. 3,044,580, H. J. Butler, discloses a mechanical system which use the torque of the brake pad housing to generate additional pressure on the brakes pedal. The brake pad housing is connected by a cable to the brake pedal. However, the feedback systems utilized do not incorporate a positive release in the loop. These systems rely upon a mechanical release of the brake pad upon the release of the brake pedal. But because this device is connected to the brake pedal, the release of the brake pedal by the user does not necessarily release this device.
This introduces a safety hazard to the user. The disengagement of the frictional member from the rotational member is not assured. The disengagement could be prevented because the device is also applying the brake pedal keeping the brake pad in engagement with the rotational member. The brake pad pressure could continue to be applied, increased by the power assist feedback loop and bring the vehicle to an abrupt, uncontrolled, and unexpected halt.
An approach to introduce a positive disconnect of the brake assistance pressure is shown in U.S. Pat. No. 3,664,468, H. Oka. This system relies upon a spring to act upon the mechanical levers rotated by the torque exerted upon the frictional member under the condition of the member engaged with the rotational member. The spring is to force the levers back to a neutral position. If the spring becomes weak, the levers will not be forced back to a neutral position. The assist pressure will not be released. Thus, the brake will stay engaged bringing the vehicle to a halt.
Some approaches to solve the problem have been to combine the mechanical feedback with a hydraulic feedback as shown in U.S. Pat. No. 3,700,075. This approach utilizes a limiter to compare the original user applied pressure to the power assistance pressure. A floating valve controlled by opposing springs will disengage the power assist pressure if it exceeds an upper threshold. However, the floating valve must be damped to avoid "hunting" during the application of the power assist pressure.
This solves the problem of continuous build-up of power assist pressure and avoids uncontrolled stops but does not solve the problem of positive disconnect of the power assist pressure as the damped floating valve must seek a position which allows the pressure on the primary brake to be released which in turn releases the pressure on the hydraulic feedback and threshold device.
Thus, there has long been a need for an arrangement to apply additional pressure on a brake pad through a feedback arrangement with a positive release action to stop the application of the additional pressure.
It is desired that the feedback system incorporate a positive disconnect of the power assist upon removal by the user of pressure from the brake control.
Further, it is desired that the feedback system give immediate response to the release of the brake control by immediately relieving the power assist feature.
It is further desired the the feedback system be adjustable to limit the amount of power assist available from the system.
It is yet further desired that the power assist system provide a measure of anti-lock prevention to prevent a "hard lock" situation which may reduce control of the vehicle during an emergency stop condition.